METHANOGENIC GENERATION OF BIOGAS FROM
SYNTHESIS-GAS FERMENTATION WASTEWATERS
As societies around the world become increasingly more dependent on fossil based fuels, the need to investigate alternative fuel sources becomes more pressing. Renewable, biomass-based carbon sources obtained from the biosphere can be gasified to produce synthesis gas, which can in turn be fermented to produce fuel-grade ethanol. A byproduct of ethanol production via fermentation is acetic acid. An optimized ethanol fermentation process should produce a wastewater stream containing less than 2 g/L of acetic acid. This is not enough acid to justify recovery of the acid; however it is a high enough concentration that treatment of the stream is required before it can be discharged. The purpose of this research was to convert the acetic acid into biogas, producing a two-fold result: removal of the acid from the wastewater stream and the production of methane, which is a valuable source of energy.
Microorganisms known as methanogens will consume acetic acid to produce methane and carbon dioxide under anaerobic conditions. The goal of this research was to optimize methane production from the wastewater stream discharged from an ethanol to syngas facility. Sludge containing methanogenic organisms was obtained from the anaerobic digester of a wastewater treatment facility and used as inoculum in batch reactors containing a synthetic acetic acid solution. Variables such as the type and amount of supplied nutrients, acid concentration, pH, cell acclimation, oxygen exposure, headspace gas composition, and agitation rate were examined. The effects of these parameters on the amount of biogas produced and acetic acid degraded were used to evaluate and optimize reactor performance. Additional experimentation further evaluating methanogenesis at low pH was also conducted using a laboratory scale semi-continuous fermentor. Finally, advanced analytical techniques were used to evaluate changes in organism population with respect to changes in reactor operational parameters. The results of this research were used to estimate kinetic parameters, develop different full-scale reactor design models, and estimate the both the cost of wastewater treatment as well as the value of the methane produced.